The invention relates generally to monitoring fluid in a line without direct fluid contact, and more particularly, relates to non-intrusively monitoring for changes in fluid properties, including the presence of air or other gas, and for fluid pressure.
In numerous medical and industrial applications, continuous in-line monitoring of a fluid is often necessary to ensure consistency of a process or to ensure safety. For example, the pressure of fluid in a line may be critical to a process. Additionally, the presence of air or other gas within a fluid or the presence of contaminants within a fluid may need to be monitored. Examples of non-medical applications for fluid monitoring can be found in the chemical process industry, where inexpensive and/or disposable fluid conduits may be required, where fluids may be present at high pressure, or where fluids which are highly caustic or highly toxic may be involved.
In the medical area, safety and cost are of great concern. Reliable and inexpensive in-line fluid monitoring without direct fluid contact is especially important in the medical area where sterilization and disposability of the fluid line are requirements. In fluid delivery systems with a positive pressure pumping mechanism, a flow path occlusion can be detected by monitoring the fluid pressure. When flow path disposability is required, fluid pressure is generally measured either by means of a disposable transducer or by means of a non-disposable transducer coupled with a disposable membrane or other compliant region arranged such that the transducer itself is not in direct contact with the fluid. Typically, a disposable transducer or specially designed transducer membrane is more expensive than a disposable fluid line alone. In regard to considerations of cost, it would be desirable to provide a pressure transducer which may be reused and disposable fluid lines which work with that transducer.
Air-in-line detection systems are used to prevent the inadvertent infusion of air into a patient's bloodstream. While small bubbles of air may have no adverse effect on a patient, large air bubbles can cause death. Methods for the in-line detection of air typically involve ultrasound or light transmission through the fluid line being monitored. The transmission characteristics of sound or light may be utilized as an indication of the presence of a gas bubble in liquid in the fluid line. Simple recognizable perturbations of the signals from such sensors may be utilized to trigger an alarm and/or halt the infusion. Such systems require that the fluid and the associated conduit be substantially transparent to the energy being transmitted. However, due to their inability to reliably distinguish between air bubbles of varying sizes, ultrasonic or optical air-in-line detectors sometimes behave erratically, falsely indicating the presence or absence of air bubbles. Typically, such sensors cannot determine the exact size of air bubbles and are configured merely to indicate the presence of air bubbles which are greater than a predetermined size.
Other apparatus capable of detecting impurities such as air within a fluid include electrochemical systems and laser doppler systems. Electrochemical systems can be extremely sensitive to specific compositional variations in a fluid, but incorporate components, such as membranes, which must be in direct contact with the fluid, thus increasing their costs in applications requiring disposability. Laser systems are at present very expensive, and still other systems cannot operate over the wide range of flow rates and fluid types required in many applications.
Hence those concerned with fluid line monitoring have recognized that it would be beneficial to provide an in-line fluid monitoring system and method which does not involve direct fluid contact with a sensor but which exhibits higher sensitivity to variations in fluid composition, including the presence of air or other gas, and which can provide an indication of the size of a gas bubble. In medical systems, there is a need for an apparatus and method which reliably and accurately detect and quantify the presence of air or other impurities in the line but at the same time are relatively inexpensive and can function with an inexpensive disposable fluid line. Additionally, in medical systems, there is a need for an apparatus and a method which can reliably and accurately measure the pressure within an inexpensive disposable fluid line. The present invention fulfills these needs.